1. Field of the Invention
The present invention relates to a method and an apparatus for forming a carved groove for a lug groove with a cutter on an outer circumferential surface of a green tire before curing.
2. Description of the Related Art
FIG. 7 shows a conventional carved groove forming apparatus 01 used for carving a carved groove for a lug groove with a cutter on an outer circumferential surface of a green tire.
A green tire G has a central axis supported horizontally. The carved groove forming apparatus 01 has a base stand 03 laid on a rail 02 so as to move toward the green tire perpendicularly to the central axis of the green tire G and horizontally (in direction X). The base stand 03 has a rotary shaft 04 projected in direction X toward the green tire G. The rotary shaft 04 is provided with a rotary disk 05 integrally.
The rotary disk 05 has a pair of rails 06 along which a slide disk 07 slides. The slide disk 07 has a cutter 08 and a photo-sensor 09. The cutter 08 stands up and falls down relatively to the slide disk according to action of an air cylinder. Carving of the green tire G by the carved groove forming apparatus 01 is carried out as follows.
Firstly, outer circumference of the green tire G is partitioned off into 36 portions, and in each portion as shown in FIG. 8, a cut shape (shown by one-dot-dash line in FIG. 8) is described on the outer circumferential surface of the green tire by marking-off.
Next, referring to FIG. 9, the photo-sensor 09 detects cut starting point A and measures distance, the base stand 03 is moved in the direction X so that the fallen cutter 08 positions at a place distant from the green tire G by a predetermined distance, then the rotary shaft 04 and the rotary disk 05 are rotated together by hand to set direction of the rail 06 so that the cutter 08 moves in direction of angle of marking-off of the cut shape.
After that, the cutter 08 is revolved to stand up by the air cylinder, and eats into the green tire G by a predetermined depth at the cut starting point A as shown in FIG. 9.
Next, slide disk is moved together with the cutter 08 along the rail 02 by hand to form the carved groove.
In case of a green tire, variation of outer diameters is large compared with variation of tread gauges. When the green tire is cured, a mold having lug groove ribs projected from the inner circumferential surface is put on the green tire with the lug groove ribs engaged with the carved grooves of the green tire, and inner pressure is applied to the green tire to press the green tire against the inner circumferential surface of the mold and form lug grooves of a product tire.
Therefore, even if variation of outer diameters of the green tire is large, the variation is dissolved by curing and the product tire of substantially true circle is obtained.
In this specification, the distance from the central axis to the outer circumferential surface of the green tire is called the outer diameter.
FIG. 9 shows a state of the green tire G in which outer diameters vary largely in particular. In this green tire G, if the cutter 08 is moved in parallel with the central axis C-C′ of the green tire G, a carved groove of uneven depth as shown by the broken line is formed, and the uneven depth remains as it is when the green tire is deformed into a state near a true circle by the above-mentioned inner pressure. Therefore, shape of the lug groove rib and shape of the carved groove are different from each other largely, so that an accurate lug groove can not be formed.
Thus, the portion to be carved of the green tire is carved to a necessary depth only at the cut starting point A, and another part of the portion to be carved can not be carved to the necessary depth because the depth is influenced by variation of the outer diameters.